E.ON Netz, “Grid Code High and extra high voltage”, E.ON Netz GmbH Bayreuth, 1 Apr. 2006, p. 15-20 requires generating plants, such as wind farms, to support the grid voltage with additional reactive current during a voltage dip. To do this, voltage control is activated in the event of a voltage dip of more than 10% of the effective value of the generator voltage. The voltage control must take place within 20 ms after fault recognition by providing a reactive current on the low-voltage side of the generator transformer amounting to at least 2% of the rated current for each percent of the voltage dip. A reactive power output of at least 100% of the rated current must be possible if, necessary. After the voltage returns to the dead band, the voltage support must be maintained for a further 500 ms in accordance with the specified characteristic.
US 2010/0176770 A1 describes a method and an electricity-generating installation, e.g. a wind energy installation, for stabilizing an electricity-distribution network. After detection of a network fault, a stabilization regulator in the installation is activated. The stabilization regulator regulates the voltage of the electrical power as a function of a feed-back voltage signal and has a step-function response that rises over time. After a predetermined time period has elapsed since the activation of the stabilization regulator, the reactive current feed is increased beyond a limit which is provided for normal operation if the network has not stabilized.
US 2009/0250931 A1 describes a method for operating a wind energy plant with a fault-ride-through functionality. In the event of a fault the converters are controlled by a control module which controls the torque, active power, reactive current and reactive power by means of command variables so as to support the grid in the event of a fault. For example, the current function for the reactive current will be determined dependent on the absolute difference between the nominal grid voltage and the grid voltage during the fault. In this way it is made sure that the wind energy plant will deliver the necessary contribution to support the grid in the event of a fault.